1. Field of the Invention
The present invention relates to a power supply system for consumable electrode arc welding wherein a consumable electrode is fed toward a base metal for arc welding and the polarity of a DC voltage applied across the consumable electrode and the base metal is alternately switched between the reverse polarity and the straight polarity, and to a method of controlling the power supply system.
2. Description of the Prior Art
Generally, the consumable electrode arc welding in which a consumable electrode (welding wire) is fed at a fixed speed for welding is reverse polarity welding, in which a DC voltage is applied across the consumable electrode and the base metal so that the polarity of the consumable electrode is positive and that of the base metal is negative.
Reverse polarity welding has advantages that the depth of penetration is large and flat beads are formed easily, because the heat input to the base metal is large. However, in welding thin base metals such as thin plates, burnthrough is liable to occur when the accuracy of the joint is low and a gap is formed between the base metals.
On the other hand, the straight polarity welding in which a DC voltage is applied across the consumable electrode and the base metal so that the polarity of the consumable electrode is negative and that of the base metal is positive is suitable for welding thin plates having a gap therebetween and requiring large reinforcement of weld, because the heat input to the base metal is small. However, straight polarity welding has problems that convex beads are liable to be formed when the accuracy of the joint is comparatively high and that lack of fusion is liable to occur depending on the shape of the joint when the welding wire is misaligned slightly with the joint.
U.S. Pat. No. 4,485,293 discloses a power source for consumable electrode AC arc welding, in which the welding mode is changed over alternately between the reverse polarity arc welding and the straight polarity arc welding to melt base metals in a desired depth of penetration to make most of the advantages of the reverse polarity welding and the straight polarity welding.
Generally, in consumable electrode arc welding employing the conventional arc welding power source, the arc is extinguished when the output voltage passes the zero point. The arc breaks off and reignition of arc is impossible when the open circuit output voltage is on the order of 80 V of the the conventional welding power source. Consequently, the arc stability is deteriorated remarkably and it is impossible to carry out stable welding.
In the short-circuiting arc welding process and power source disclosed in U.S. Pat. No. 4,485,293, the same polarity is maintained during an arc period and the polarity is changed over during a short-circuiting period to obviate unstable welding attributable to the extinction of the arc. Furthermore, this welding source is able to control the welding mode in an optional state between the reverse polarity arc welding and the straight polarity arc welding by varying the ratio between them on the basis of the frequency of short circuit and to control the heat input to the base metal and the melting rate of the welding wire.
However, since the opportunity to change over the polarity is limited only to the short-circuiting period in such a welding process, the polarity duration ratio between the reverse polarity arc welding and the straight polarity arc welding deviates greatly from a set polarity duration ratio when the short-circuiting period and the arcing period vary due to the vibration of the molten pool and the wire extension. Furthermore, the control of the polarity duration ratio between them is impossible in a high current range in which the frequency of short circuit is small.